Recent evidence indicates the importance of the innate immune system in both graft rejection and the induction of tolerance in solid organ transplantation. However, the specific cell types and molecules of the innate immune system involved in these processes have not been determined. We have begun to define natural killer (NK) cell activation in the context of solid organ transplantation. We have shown that NK cells comprise the majority of liver infiltrating cells early post-transplantation. These NK cells are functionally active and producing substantial amounts of IFN-y. Furthermore, depletion of NK cells results in decreased levels of IFN-y and prolonged graft survival. Based on these data we suggest a model in which surgical stress and ischemia/reperfusion injury stimulate the liver allograft to induce the early expression of several chemokines that direct the recruitment of recipient-derived NK cells into the allograft early after transplantation. IFN-y produced by NK cells further induces the recruitment of activated lymphocytes to the graft thereby augmenting effector function and graft damage. We hypothesize that the innate immune system, specifically the activation of NK cells, influences graft outcome post transplantation. This hypothesis will be tested using a rat orthotopic liver transplant model to examine the role of NK cells during graft rejection and long term graft survival. In the first Specific Aim we will determine the role of NK cells in graft outcome following liver transplantation. We will: 1) analyze the role of NK cells post-transplant, 2) determine the effect of NK cell depletion on chemokine and IFN-y production and cytotoxicity post-transplant, 3) analyze the establishment of mixed chimerism in the absence of NK cells and 4) determine the effect of Immunosuppressive agents on NK cell effector function. The goal of the second specific aim is to determine the functional significance of specific NK cell receptors in graft outcome. Unique reagents we have developed and assembled, will be used, to specifically address the expression and functional significance of the rat NK cell receptors in the context of solid organ transplantation. Specifically we will: 1) determine the expression pattern of NK cell activation receptors after transplant, 2) determine if signaling through NK cell receptors induces cytokine production and cytotoxicity, and 3) examine the functional outcome of blocking NK cell receptors in a transplant model. Our studies will specifically define the functional significance of NK cells in both allograft rejection and acceptance an will lead to new approaches to induce tolerance to liver allografts.